The European patent application published under the number 0 651 409 A1 describes a mechanism of this type. In that embodiment, all the teeth of the small toothed-wheel and the first tooth which follows, in the predetermined direction, the gap left by the discontinuity in the teeth of the large toothed-wheel are configured in such a manner that for each of them their flanks converge towards a single edge. The purpose of that measure is to prevent the toothed wheels jamming. Unfortunately, it turns out that the friction of an edge of a small toothed-wheel on the leading flank of the first tooth following the gap in the large toothed-wheel gives rise to power losses of greater or lesser size due to the rubbing. Such losses which depend on the position of the small toothed-wheel give rise to variations in the time required for engaging the switchgear and also to non-negligible wear of the first tooth following the gap in the large toothed-wheel. If the mechanism is used to control a switch that is designed to reduce network surges by being engaged synchronously with the voltage on the network, then such randomly-appearing variations are unacceptable.
European patent No. 0 294 561 A2 describes a drive mechanism of a similar type, coupled to a disconnector and in which the first tooth of the large toothed-wheel situated immediately after the gap is radially retractable against the bias of a compression spring. If this tooth comes into abutment against the tip of a tooth of the small toothed-wheel at the beginning of the engagement process, it retracts radially, compressing the spring that is associated therewith. As a result, the tooth can slide without jamming on the edge of the corresponding tooth of the small toothed-wheel. Once it has gone past this edge, it engages in the space following the tooth of the small toothed-wheel. Nevertheless, there is a major risk of the retractable tooth continuing to entrain the large toothed-wheel by friction, in spite of being progressively retracted into the housing which contains the spring. Under such circumstances, the second tooth following the gap provided in the large toothed-wheel, and which is not retractable, can come into abutment against the edge of a tooth of the small toothed-wheel and jam the entire gear mechanism. This probability is high since the small toothed-wheel is decoupled from its drive by a freewheel coupling, and can therefore turn very easily.